In modern production techniques, particle board with a wide range of densities and in a variety of thicknesses may be produced for various purposes, ranging from load-carrying structural uses to insulating or decorative purposes.
The term "particle board" is here used to refer to a coherent and self-supporting product formed by the compaction of heat and pressure of a mat of particles, generally of wood, with intrinsic binder substances or with synthetic resin binders which can be added to the particles.
The particles may be wood chips so made upon comminution of the wood that they are relatively elongated and thus can be considered to have an orientation.
The particle-board mass can be pressed in single-level presses or in multiplaten presses, directly upon the platen surface or on press underlays, e.g. screens which can impart textures or patterns to the underside of the particle board.
During compression, the upper surface of the board can be embossed so as to have a particular texture and the board can be pressed to extremely high densities for use as load-supporting members or relatively low densities for use in applications in which load-bearing may not occur.
The particle board is also frequently termed composition board, and may be used directly or may be laminated to veneer or other decorative layers, can be provided with synthetic resin decorative or protective foils or can be coated depending upon the ultimate use.
In all cases, however, the precursor of a particle board is a mat, generally of rectangular configuration and a thickness which may be several times the thickness of the board into which the mat is compacted.
This mat is, at best, only loosely coherent and consists basically of a flat pile of the particles or wood chips.
It is known to form such mats by depositing the particles in a more or less uniform layer upon a receiving surface, such as a conveyor, which can carry the mats away from the layer-forming station to a station in which the layer is divided transversely to the longitudinal dimension of the layer so that discrete mats are produced.
The individual mats may be transferred directly or indirectly to the press, indirect transfer usually involving careful deposition of the mat upon a press underlay or tray which serves to carry the mat onto the press platen.
In the formation of mats or layers of particulate materials, especially wood chips, it has been found to be advantageous during the layer formation to orient the elongated particles in a transverse direction, i.e. such that the major dimension of the chips lies transverse to the conveyor movement at least predominantly.
Transverse orienters can be associated with the dispenser for depositing the particles upon the layer-forming surface.
Thus it is known to provide, for the manufacture of particle board, an apparatus or device for depositing the chips upon a continuously moving layer-forming surface, e.g. the aforementioned conveyor, which comprises a transverse orienter which moves continuously along an endless path and which has a lower portion overlying the conveyor and movable in the direction of movement of the conveyor. A dispenser for the particles is disposed above this transverse orienter.
As the layer is formed on the conveyor, the transverse flights engage the particles of the deposited mass and impart a preferential transverse orientation to the particles.
A system of this type is described in German patent document (Open Application) DA-OS 25 23 515 and is illustrated in FIG. 5 thereof.
In this system, the transverse orienter is a drum having a closed periphery and the flights project radially from this drum.
The flights thus form chambers which are bottomed by the closed periphery of the drum and which rotate about the axis thereof.
While this system has been found to be effective in large measure, it also has certain problems which can be attributed to the drum and chamber construction. For example, the resulting layer is not as uniform as is desired and the homogeneity of the transverse orientation may vary depending upon the location and the nature of the engagement of the flights with the particles.
The particle board which is formed from the mats is not isotropic in the transverse direction and there are regions in which the density may vary or other properties of the particle board may fluctuate across the board. In some cases the result is warping while in other cases weakened portions of the board can be detected.
It has been proposed to overcome these disadvantages by providing between the drum with its flights and the layer-receiving conveyor, a further roller which is intended to smooth out the layer as it is formed and in part overcome the irregularities which are inherent in the use of the pocketed drum surface formed by the flights.
While the layers resulting from this apparatus are indeed smoother and more uniform in appearance, the added roller is found to distribute the transverse orientation so that the desired particle orientation effect is not achieved and many of the problems with lack of isotropism or strength remain.
With pocketed drums of this type, when the dispenser drops particles from above onto the drum so that the drum pockets collect these particles and carry them onto the conveyor, the particles are deposited in clumps which also interfere with the uniformity of the resulting layer.